Ruthenium catalysts are used in hydrogenation of unsaturated organic compounds, hydrogenation of olefins, ketones, aldehydes, etc. and, in particular, hydrogenation or partial hydrogenation of the nuclei of aromatic compounds by taking advantage of the high activity and specific reaction selectivity of the ruthenium catalysts.
In general, typical examples of cause for the decrease in activity of a catalyst used in hydrogenating an unsaturated organic compound are a physical change of the active site of the catalyst itself caused by reaction circumstances such as reaction temperature and reaction heat (e.g. sintering), or the accumulation of a catalytic poison (e.g. a sulfur compound or a foreign metal). In order to avoid these causes, some measure for controlling the reaction temperature or preventing contamination with the catalytic poison is widely and industrially taken.
The same activity decrease phenomenon as above occurs in the case of ruthenium catalysts. In particular, as to the decrease of their activity caused by a catalytic poison, JP-A-60-255738 describes an example of poisoning by a sulfur compound, and JP-A-62-67033 describes poisoning by iron. JP-A-62-65751 discloses an example of method for regenerating a ruthenium catalyst poisoned by a sulfur compound.
However, it has been confirmed that a ruthenium catalyst repeatedly or continuously used in hydrogenating an unsaturated organic compound has a decreased activity which is considered to be due to a cause utterly different from the above-mentioned physical change or poisoning of the catalyst. Although theoretically clear explanation of this phenomenon is difficult, it can be speculated that the amount of some reaction inhibitor produced by the interaction between hydrogen and the ruthenium catalyst increases with the lapse of time under the reaction conditions, for example, because an activity decrease larger than that estimated from the physical change and poisoned condition of the catalyst is observed and the reaction is carried out in the presence of hydrogen.
As a method for recovering the ruthenium catalyst activity decreased by the above cause, JP-A-1-159059 discloses a method of bringing the ruthenium catalyst into contact with oxygen in a liquid phase, and JP-A-3-68453 discloses a method of maintaining the catalyst at a hydrogen partial pressure lower than that at the hydrogenation and a temperature not lower than a temperature lower by 50.degree. C. than the hydrogenation temperature.
However, although these conventional methods are effective when the decrease rate of the activity relative to the initial activity is low, they are not sufficiently effective for a ruthenium catalyst decreased in activity to 60% or less of the initial activity by repeated or continuous use. Therefore, there is desired a more effective method for recovering the activity of such a ruthenium catalyst.
JP-A-7-178341 discloses a method comprising heat-treating a silica-supported ruthenium catalyst used in hydrogenation at a temperature of 60.degree. C. or higher in an oxygen-containing gas and then at a temperature of 20-200.degree. C. in a hydrogen-containing gas, but does not describe the regeneration of a ruthenium catalyst in a liquid phase.